Involvement of cathepsins B and H in lysosomal degradation of horseradish peroxidase endocytosed by the proximal tubule cells of the rat kidney: I. Histochemical and immunohistochemical studies

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Abstract
Immunohistochemical localizations of horseradish peroxidase (HRP) and cathepsins B and H in the lysosomal system of the rat kidney proximal tubule cells were investigated during the internalization and degradation of HRP by these cells. At fixed intervals after intravenous injection of HRP, the kidneys were fixed by perfusion. Five to fifteen minutes after this injection, endocytosed HRP was detected in fine granules beneath the brush border. Thirty minutes later, it accumulated into large aggregates of granules (phagosomes), where both enzymatic and antigenic activities appeared to cause HRP to be lost rapidly. After 1 hour, the aggregates of phagosomes increased in number and size but soon began to decrease gradually. In the early stage, cathepsins B and H were not detected in the apical cytoplasmic granules. However, they were present in the lysosomal compartments, together with the HRP. The large aggregates of phagosomes showed a patchy reaction, especially approximately 30 minutes after injection, whereas other medium‐sized phagosomes were stained heavily. Aggregates of phagosomes were present only in the S1 segment. These results suggest strongly that cathepsins B and H are involved in the cellular degradation of endocytosed HRP.
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